A single Radeon R9 390X isn't that impressive as it's just a rehashed R9 290X, but two of the new R9 390X cards in CrossFire represents great performance. With a price of $429 or so per card, you're looking at $850+ for this setup which is why we've included the performance of the NVIDIA GeForce GTX 980 Ti which sells for $649 and above (depending on the brand).
I'm more than impressed with what AMD is capable of here with two Radeon R9 390X cards in CrossFire at 4K, but I think the real excitement is going to be next week when the Radeon R9 Fury X arrives... we'll be running it through its paces, seeing just what HBM brings to the table.